Turnable coaxial cavity magnetron



March 25, 1969 now ETAL 3,435,284

TURNABLE COAXIAL CAVITY MAGNETRON Sheet Filed Dec. 28, 1965 EDWARD TROBE/P75 BY z a PLUM was .47'70RNEY March 25, 1969 E. 'r. DOWNING E TAL3,435,234

TURNABLE COAXIAL CAVITY MAGNETRON Filed Dec. 28, 1965 Sheet ,2 of 2 j Sj 98 5 1/111 5 was j 4 5-//6 //4 x i 5. 2

I l HVVENTORS EDWARD 7'. DOWNING ROBERT E PLUMRIDGE United States Patent3,435,284 TURNABLE COAXIAL CAVITY MAGNETRON Edward T. Downing,Winchester, and Robert F. Plumridge, Lexington, Mass, assignors toRayethon Company, Lexington, Mass, a corporation of Delaware Filed Dec.28, 1965, Ser. No. 516,901 Int. Cl. HOlj 25/50 US. Cl. 31539.61 6 ClaimsABSTRACT OF THE DISCLOSURE The present invention relates generally tomicrowave oscillator devices of the magnetron type having a coaxialresonant cavity disposed externally or internally of the anode resonatorsystem and more particularly to a new and novel tuning structure forsuch devices.

A well known microwave frequency generator is the crossed-fieldmagnetron device incorporating a central cylindrical cathode encircledby a plurality of anode cavity resonators. An external magentic field isapplied parallel to the axis of the cathode member while an electricfield is established transverse thereto between the cathode and anodemembers. Electrons traverse the interaction region between the cathodeand anode in an orbital path and are coupled sequentially to the anodecavity resonators to generate high frequency oscillations in theso-called 1r mode which are then coupled to the load. Such devices havebeen rendered as either tunable or fixed tuned structures for pulsed orcontinuous wave operation,

In late years a more efiicient magnetron device has been developed whichenhances the stability of the generated frequencies. Such devicesincorporate a separate annular resonant cavity system coupled coaxiallyto a conventional anode-cathode interaction system. In one configurationthe annular cavity resonator is mounted externally to the anode memberwhile in the so-called inverted configuration the cavity resonator iscentrally disposed with respect to the anode-cathode system whichresults in the provision of increased cathode emission surface area.

Tuning of the coaxial cavity magnetrons has been heretofore accomplishedby means of an annular tuning ring member displaced parallel to the tubeaxis or essentially in the direction of the applied lines of force ofthe magnetic field. The frequency of the output oscillations is adjustedby moving the tuning ring coaxially in relation to the anode member.Such tuning structures closely resemble a flat doughnut configurationand conventionally such tuning structures, due to the disposition withinthe cavity, are fabricated of a metallic material. Displacement of thetuning member simultaneously in all quadrants of the annular cavityresonator gives rise to some rather elaborate actuating structures whichare both clumsy and expensive. Additionally, spurious oscillation modesare excited within the cavity resonator to reso nances created betweenthe tuning member and the outer walls of the outer cavity resonator.

Another development has evolved in the design of microwave oscillatorsand accompanying systems incorporating the frequency agility conceptwhich involves rapid changing of the operating frequency by continuous-1y rotating a tuning member and selectively correlating such rotationswith the pulsing cycles of the high frequency oscillations. With thistechnique a complete tuning cycle is covered with each rotation of thetuning member. At speeds in the range of 2,000 r.p.m. or higher and asingle anode resonator, thirty-three complete tuning cycles every secondmay be achieved. It the pulse repetition rate and the rotation rate issynchronized a constant output frequency on each pulse will begenerated. Variation of the speed of rotation in relation to the pulsingwill produce a great number of combinations of output frequencies andwith higher rates of speed more than one frequency cycle can occur inthe interpulse interval. Furthermore, variation of the pulse repetitionrate while maintaining a constant rotation of the tuning member mayprovide still another large number of combinations of outputfrequencies, Rotary tuning to achieve frequency agility has achievedimportance in the field of electronic countermeasures to preventanalysis of transmitted signals for the purpose of jamming.

A possible solution to the incorporation of frequency agility by cyclictuning in coaxial cavity magnetron devices would be to oscillate themetallic tuning ring members up and down at a rapid rate by somemechanism such as a hydraulic actuator. The disadvantages in theprovision of such a system, particularly when coupled to a highlyevacuated electron tube, is immediately evident. The uniformdisplacement of such tuning ring members will also give rise to somecomplex mechanical problems and the additional required structure may beprohibitively costly.

Accordingly, it is an object of the present invention to provide meansfor the rotary tuning of coaxial cavity magnetrons to provide for rapidcyclical variation of the operating frequency over the tuning range.

Still another object of the present invention is the provision of arotary tuning apparatus which provides for rapid perturbation of theelectric fields of a coaxial annular cavity resonator of the external orinternal type for crossed field magnetron oscillator devices.

Another object of the present invention is to provide for capacitivetuning of coaxial cavity magnetrons.

The invention broadly stated comprises the incorporation of a tuningmember in a coaxial cavity magnetron of the type hereinbefore describedwith the tuning element coaxially with relation to the axis of thecavity resonator. Rotation of the tuning element at high speeds in aplane transverse to this axial direction will result in the tuningelement in one angular position being placed in the low electric fieldregion while a position rotated thereto places the tuning element in theregion of the high electric field. In one embodiment envisaged in thepractice of the present invention a pair of parallel diametricallyopposed elements are provided which when rotated will achieve onecomplete cycle of the tuning range. Another embodiment of the inventionprovides for the eccentric disposition of parallel tuning elementssimultaneously in the high and low electric field regions indiametrically opposed quadrants of the cavity. Still another embodimentof the invention incorporates a single tuning element pivotally disposedto simultaneously traverse the low and high electric field regions uponrotation or angular displacement to achieve dither tuning.

A feature of the present invention is the provision of a tuning elementof a dielectric material having a high dielectric constant and lowdielectric losses. Illustratively, such materials as alumina or sapphireare suggested in the practice of the invention. With the employment ofthe dielectric continuously rotated tuning member capacitive tuning ofthe coaxial cavity magnetron is achieved.

In addition to the foregoing, other objects, features and advantageswill be readily apparent after considera- 3 tion of the followingdetailed specification and reference to the accompanying drawings, inwhich:

FIG. 1 is a vertical cross-sectional view of an illustrative embodimentof the invention;

FIG. 2 is a cut-away exploded view of a portion of the embodiment shownin FIG. 1 as indicated by the general reference numeral A;

FIG. 3 is a diagrammatic view illustrative of the disposition of theelectric and magnetic fields in oscillators of the coaxial cavity type;

FIG. 4 is a vertical cross-sectional view of a magnetron oscillator ofthe integral cavity type incorporating the embodiment of the presentinvention;

FIG. 5 is a diagrammatic view of an alternative embodiment of theinvention; and

FIGS. 6 and 7 are a plan view and cross-sectional view respectively ofstill another illustrative embodiment of the invention.

Referring to the drawings, the illustrative embodiment shown in FIGS. 1and 2 comprises a magnetron oscillator 2 having a cylindrical cathode 4centrally disposed within a cylindrical anode system including aplurality of cavity resonators 6 defined by inwardly disposed vanemembers 8 secured to cylinder member 10. Magnetic pole piece members 12and 14 are disposed on opposite sides of the anode system along thecentral axis of the over-all tube. Substantially C-shaped permanentmagnet members 16 are disposed in a position extending at right anglesto the surface of the drawing as viewed by the reader. The dispositionof the magnet as shown therefore is 90 out of its actual position and ismerely illustrated in this manner for the purposes of the specificationto include all applicable structure necessary for the operation of theover-all device. The magnet system described will provide lines of forceWithin the anode-cathode interaction region indicated generally by thenumeral 18 as well as the external cavity resonator system to behereinafter described in a direction substantially parallel to the axisof the cathode 4.

The cathode is supported by a cylindrical member 20 secured in theconventional manner to metallic collar 22 insulatedly secured by meansof dielectric member 24 to metallic annular member 26 and pole piece 14.Outer terminal member 28 provides for the energizing of a heater (notshown) within the cathode 4 by suitable high voltages. The remainder ofthe conventional magnetron oscillator structure comprises cooling fins30 and exhaust tubulation 32 for evacuation of the internal atmosphere,output Waveguide section 34 including a conventional transformer sectionand mounting plate 36 for supporting the tube in a socket arrangementwith or without a viscous fluid coolant circulated in the regionadjacent the terminal member 28 and the cathode support structure.

An auxiliary annular coaxial output cavity resonator 38 is defined byouter cylindrical wall member 40 and is coupled to certain anoderesonators by slots 42 to permit the introduction of microwaveoscillations. There thus are two co-planar conductive walls 10 and 40extending parallel to the axis of the anode system. The dimensions ofthe cavity resonator 38 are selected to provide resonant frequenciesover a predetermined frequency band with the TE mode of oscillationpreferred in such coaxial cavity magnetrons.

In accordance with the teachings of the invention means are provided forthe varying of the resonant frequencies including in this embodiment anonconductive tuning element 44 having a pair of parallel diametricallyopposed members 46 and 48 disposed parallel to the axis of the anodesystem and the co-planar walls. Support member 50 provides for thedisposition of the elements 46 and 48 as well as coupling to thedisplacing means of the tuning element in the manner to be hereinafterdescribed. A shaft 52 extends the length of the pole piece 12 tocommunicate with the actuating means indicated generally by the numeral54 and is supported throughout its length by bearings 56, 57 and 58.These bearings may be fabricated of the material sold under thetrademark Graphitar which is suitable for use as a bearing surfacewithin an evacuated envelope. The tuning element is actuated by rapidrotation thereof to achieve rotary tuning and frequency agility in themanner described in co-pending patent application Ser. No. 248,179,filed Dec. 24, 1962 by Robert E. Edwards and assigned to the assignee ofthe present invention, now abandoned. A relatively small fractionalhorsepower motor 60 is mounted outside the tube envelope with torqueapplied by the so-called magnetic coupling method. Hence, motor 69drives shaft 61 which engages housing 62 mounted substantiallyconcentric with the axis of the over-all tube and supported for rotationby means of bearing 64. The interior of housing 62 comprises a pluralityof members of a magnetic material 65 which substantially encirclecylindrical body member 66 and will be referred to as the drive magnet.Another bearing member 68 is disposed at the lower part of housing 62 tosupport the rotation thereof. The torque of the rotating drive magnet istranslated to a follower member 70 secured to the tuning element shaft52. Fol lower member 70 is provided with magnetically permeable materialto thereby couple magnetically to the drive magnet 65. The tuningelement 44 is thereby rotated continuously in any desired direction withthe speed of rotation selected to provide the unique features of rotarytuning to cycle the output frequency over the frequency band ofoperation at exceedingly high rates of speed in the order of severalthousand r.p.m.s and higher in a sawtooth fashion.

In order to provide an indication of the approximate frequency of themagnetron device to set the receiver local oscillator of a system at apredetermined corresponding frequency, a feedback transducer 72 may beprovided utilizing a plurality of meshing sets of capacitive plates 74with the capacitance variation as a function of angular rotationadjusted to resemble the frequency tuning curve of the magnetron. Oneset of capacitive plates is attached to the tuning shaft 52 while theother set remains stationary. Further details on the feedback transducer:arrangement are available in co-pending patent application Ser. No.428,392, filed Jan. 27, 1965 by Robert E. Edwards and assigned to theassignee of the present invention, now Patent No. 3,379,925 issued Apr.23, 1968. This arrangement is also claimed in the referenced applicationand forms no part of the invention as claimed herein.

Referring to FIG. 3 there is shown in diagrammatic form the dispositionof the crossed electric and magnetic fields in a coaxial outputresonator of the device under consideration operating in the TE mode ofoscillation. The distribution of the electric fields is indicated by thecircular solid lines 76 which traverse the cavity in a plane parallel tothe axis of cylinder member 78 while the outer cylindrical member isindicated by the numeral 80. The coupling slots 82 are illustrativelyshown to provide for the communication of the oscillations generatedwithin the anode-cathode interaction region 84 to the cavity areadesignated 86. The magnetic lines of force are directed radially in thismode of oscillation and are indicated by the dotted lines 88. Wetherefore have a cavity defined by planar conductive members 78 and andthe point of minimum electric field intensity is in the region adjacentto the respective conductors 78 and 80 while the point of maximumintensity is in the central part of the cavity region. In accordancewith the features of the invention the tuning element 44 comprises anonconductive dielectric rod of a material such as alumina or sapphirehaving a high dielectric constant and low dielectric losses. The tuningelement is rotated through 360 in one quadrant of the over-all cavity 86to thereby perturb the electric field sequentially through the points ofminimum and maximum electric field intensity. With the tuning elementdisposed in the position indicated by the numeral 44a and the dottedlines the dielectric material is disposed in the region of maximumelectric field intensity. A 90 rotation to the position indicated 44band the solid line places the dielectric tuning elements in the regionof the low electric field. As a result an alternation every 90 occursbetween the point of minimum and maximum electric field intensity andemployment of the nonconductive material results in capacitive tuning ofthe cavity resonator. This perturbation of the electric field at rapidrates of speed facilitates the cycling of the frequency through thepredetermined frequency band and in actual embodiments tuning ranges ofbetween 6 percent to 8 percent have been realized with this tuningarrangement.

In FIG. 4 another magnetron device is illustrated of the type referredto as the inverted oscillator wherein a hollow anode cylinder 90provides a central coaxial output cavity resonator 91. Extendingradially outward from cylinder 90 are a plurality of vane elements 92 todefine therebetween anode resonant cavities. One end of cavity 91 issealed 011 from the atmosphere by a dielectric window 93 whichcommunicates with the output waveguide 94 to enable the microwaveoscillations from the magnetron structure to be coupled out to autilization load. Magnetic pole piece members 95 and 96 encircle theanode cylinder 90 and concentric insulating discs 97 and 98 are providedat the outer ends to vacuum seal the over-all device. A cathode supportmember 99 is disposed between the inner ends of pole pieces 95 and 96with the emissive cathode 100 mounted adjacent the anode resonators toprovide an enlarged ernissive area. Slots 101 couple the anoderesonators to the central internal output cavity resonator 91. In thisembodiment the over-all height of the cavity resonator 91 is an integralnumber of one-half wavelengths long to provide essentially a fullwavelength cavity or greater. Slots 101, therefore, alternately coupleto one-half of the internal cavity in the manner shown with the slotsdisposed adjacent to the top and bottom walls of the cavity resonator.In this configuration a support member 102 is sealed at one end of theanode cylinder 90 and it is through this member that the tuning meansmay be introduced through a passageway suitably vacuum sealed at theouter end.

The tuning element 103 is positioned eccentrically or adjacent to thewalls of cylinder 90 and is in turn as in the previous illustrationconnected by means of shaft 104 to the motor actuating means andmagnetic coupling arrangement. Tuning element 103 comprises dielectricrods disposed parallel to the axis of the cylinder and is adapted forrotation in a plane extending normal to the cylinder axis parallel tothe disposition of the electric fields within the cavity resonator.Further details relative to the con struction of the embodiment shownmay be had by reference to co-pending application Ser. No. 387,572,filed Aug. 5, 1964 by Edward T. Downing et al. and assigned to theassignee of the present invention, now Patent No. 3,289,036 issued Nov.29, 1966.

Referring to FIG. 5, another method of capacitively tuning a coaxialcavity resonator is shown by means of an explanatory diagram. In thisalternative embodiment the nonconductive tuning element 105 is pivotallysupported by suitable means 106 and adapted to be angularly displaced bymeans of push-pull rod 107 within cavity 108 fabricated in a mannersimilar to the configuration shown in FIGS. 1 and 2 with the vanemembers 109 and slots 110. In this embodiment the method of dithertuning may be practiced by positioning the nonconductive tuning elementsinitially in the position of the lowest electric field intensity andthen displacing same a small angular dis tance to the position indicatedby the dotted line and numeral 105:: to the position of the maximumelectric field intensity. Lateral displacement of the rod 107 willprovide the relatively small tuning range required in this method oftuning which achieves approximately 3& of 1 percent of the over-allfrequency band.

Still another alternative embodiment is illustrated in FIGS. 6 and 7comprising eccentric disposition of nonconductive tuning elements indiametrically opposed quadrants of an output coaxial cavity resonator120 coupled by slots 111 to the anode-cathode interaction regionindicated generally by the numeral 112. The output cavity resonatorillustrated is of the external cavity type defined between concentricannular members 113 and 114 to provide the planar parallel conductivesurfaces. In the quadrant indicated by the numeral 120A tuning element115 is in the position of the minimum electric field intensity andsimultaneously element 116 within quadrant 120B is also in the similarlow intensity electric field region. A circle of rotation revolvingabout an axis indicated at 117 or eccentrically disposed with respect tothe axis 118 of the inner anode cylinder 113 results in the displacementevery of the tuning elements to the position of higher field intensityindicated for the purposes of this illustration as a and 116a. Anothermethod of practicing the invention is therefore disclosed to provide forperturbation of the electric field of a coaxial cavity magnetron.

While several alternative embodiments have been illustrated anddescribed herein it will be evident to those skilled in the art that asingle element tuning member may be rotated through the maximum andminimum electric field regions as well as being laterally displaced asshown in FIG. 5. Further, the high speed frequency agility offered bythe rotary tuning means employing a nonconductive element has thusprovided an effective oscillator device with means for cycling of theresonant output frequencies over the band coincidentally with the pulserepetition rate. By means of altering the speed of rotation manymultiple output frequency signals may be generated for random operationto prevent jamming of use in other countermeasure applications.Additionally, cavity resonators may be capacitivel tuned where othermodes of oscillation are generated by perturbing the electric field withsuitable nonconductive elements disposed in the most suitable manner totraverse the maxima and minima electric field intensity pointssequentially in a complete rotational cycle in a plane designed toaccomplish this traversal.

What is claimed is:

1. A microwave magnetron oscillator comprising:

a cathode member;

a cylindrical anode member defining a plurality of radially disposedcavity resonators disposed adjacent to said cathode member;

transverse electric and magnetic fields;

an auxiliary annular output cavity resonator disposed coaxially withrespect to said anode member;

means for coupling microwave oscillations from said anode cavityresonators to said auxiliar output cavity resonator;

means for capacitively tuning the resonant frequency of said auxiliaryoutput cavity resonator including a nonconductive tuning elementdisposed within said resonator so as to intersect the maximum andminimum region of electric field intensity;

and means for displacing said tuning elements in a direction normal tothe axis of said anode member.

2. A microwave magnetron oscillator according to claim 1 wherein saidauxiliary output cavity resonator is disposed externally with respect tosaid cathode and anode members.

3. A microwave magnetron oscillator according to claim 1 wherein saidauxiliary output cavity resonator is centrally disposed within saidanode member.

4. A microwave oscillator according to claim 1 wherein said tuningelement comprises parallel diametricall disposed rod members of adielectric material rotatably mounted within said output resonator.

5. A microwave oscillator according to claim 1 wherein said tuningelement comprises a dielectric member pivotally disposed within saidoutput resonator and adapted to be angularly displaced from an initialposition parallel to the axis of said anode member to result inincremental variations of the resonant frequency of said outputresonator over a predetermined frequency band.

6. A microwave oscillator according to claim 1 wherein said annularauxiliary output cavity resonator comprises spaced parallel planarconductive walls mounted externally with respect to said cathode memberand anode resonators and said tuning element comprises a dielectricmember disposed parallel to said walls within one quadrant of saidresonator and a similarly disposed member is positioned in thediametrically opposed quadrant of said resonator.

References Cited UNITED STATES PATENTS 2,444,066 6/1948 Rostas 333-83 82,492,996 1/1950 Haxby 33383 2,500,430 3/1950 Pierce 315-39.61 X3,247,421 4/1966 Backmark 31539.61

OTHER REFERENCES Microwaves: New Magnetron Shifts Frequency Fast? byEdwards, Electronics, April 1964, pages 76 to 80 relied upon, copy inGp. 255, 315-3951, TK 7800 E58.

ELI LIEBERMAN, Primary Examiner.

S. SHATMON, JR., Assistant Examiner.

U.S. Cl. X.R.

6/1948 Tompkins 33383 15 31539.77; 331-90

